This invention relates to a flaw detection system for the detection of defects on a moving web or sheet of material, and more particularly to a method and apparatus for such a system which provides a repeat defect detection system. A more general purpose circuit is incorporated which has general application to other systems in the automatic inspection field.
In U.S. Pats. Nos. 3,900,265 and 3,980,891, which are assigned to the assignee of the present invention, flaws or defects are detected on the surface of the material being examined by repeatedly scanning a source of radiation, such as a laser beam, across the surface of a moving web or sheet of material. The laser light is reflected, transmitted, or scattered from the material being examined, depending on the characteristics of that material, which light is picked up by a receiver having a suitable detector such as a photomultiplier tube. At any instant of time during the scan the photomultiplier output varies with the reflectivity, transmissivity, or scattering properties of the spot of light on the material upon which the laser beam is impinging. Deviations in the amount of light or radiation coming from the material being examined from what would be considered normal for such a material provide a means for indicating flaws or defects on the material. In U.S. Pat. No. 3,980,891, a system is provided to readily determine on what portion of the sheet of moving material the flaws occur by dividing the web of material into strips or lanes and counting the flaws and processing the flaw information with respect to the number of flaws occurring on a certain lane, strip or segment of the sheet. This has been referred to as a data-routing function. Such systems have proved excellent for a number of applications. However, for certain applications where it is desirable to detect and identify a given flaw which repeats itself on predetermined travel distances on the web of material under examination, and which discriminates against random flaws caused by noise or other inconsistencies, more sophisticated processing techniques and methods are required. For example, suppose that the web of material being examined passes over a conveyor system using a plurality of rollers having different circumferences positioned at various locations along the path of travel of the web of material. If one of the rollers accumulates some dirt or foreign matter, the chances are this will provide a flaw on the surface of the material, such as photographic film, and the flaw will repeat on the material on each complete revolution of the roller. By identifying such a repetitive flaw after a predetermined amount of travel of the web of material, the system could be shut down and the roller cleaned at the precise point where the problem has been identified, thus eliminating the problem without having to shut down and clean the entire system.